CONTEXT: Thyroid hormone impacts on the cardiovascular system. (Subclinical) hyperthyroidism results in sympathovagal imbalance due to decreased vagal tone. However, conflicting data have been reported on the effects of hypothyroidism on the activity of the autonomic nervous system (ANS). In hypothyroidism, both increased and decreased sympathetic activity and increased vagal tone have been found. OBJECTIVE: To study the effects of acute short-term overt hypothyroidism and thyroxine replacement therapy on the ANS by measuring urinary excretion of catecholamines and heart rate variability (HRV). DESIGN: Prospective study. SETTING: University hospital. PATIENTS: We studied 11 patients, previously treated with thyroidectomy for differentiated thyroid carcinoma, during hypothyroidism caused by cessation of thyroxine substitution for 4 weeks and during thyroxine replacement therapy, and 21 matched healthy controls. MAIN OUTCOME MEASURES: The activity of the ANS was assessed by measuring urinary excretion of catecholamines and HRV at rest and during a challenge of the ANS by a mental stress test. RESULTS: Urinary dopamine excretion was significantly lower during hypothyroidism. Although in the patients total variability was unchanged, HRV analysis showed a significantly lower low frequency/high frequency ratio, indicating sympathovagal imbalance with sympathetic withdrawal. The mental stress test in the patients resulted in a significant increase in heart rate to the extent of 16-18%. This response was not different between the hypothyroid state and during thyroxine replacement therapy suggesting that cardiovascular reflexes in these patients remain intact. CONCLUSION: Acute short-term overt hypothyroidism results in sympathovagal imbalance with sympathetic withdrawal, with preservation of the cardiovascular reflexes to (mental) stress.